Portable communication tools include a portable telephone, a PDA, a foldable electronic notebook, a mobile computer, etc., which require a variety of functions such as transmission and reception of a mail, voice recognition, loading an image by using a miniature camera and the like. Accompanying the diversifying functions, an increasing amount of information is displayed, and it has been urged to improve a level of visibility and quality of image. In recent years, a portable telephone which is a representative example of the portable communication tools having a display of a size in excess of two inches and a resolution that meets QVGA (Quarter VGA) has been came in the market.
The display of the portable telephone is used in various environments such as outdoors, indoors, etc. The visibility of the display varies depending upon the ambient brightness (i.e., the intensity of illumination on the side of the display portion: note that, the intensity of illumination is a flux of light received by a unit area of a surface irradiated with light: here, the display portion is illuminated by the ambient light, and the intensity of illumination on the side of the display portion stands for the ambient brightness). For example, under the sunlight or under the illumination, the displayed information can be perceived despite the brilliance of the display is low due to the ambient brightness. In a dark place, the displayed content can be less perceived when the display is too bright compared to the surrounding, and the display can be easily perceived when the brilliance of the display is slightly lowered. While, in a gloomy place in between, it is desired that the brilliance of the display is high for perceiving the displayed contents on display.
Since the visibility of the display differs depending upon the ambient brightness, there has been proposed a method of improving the visibility of the display by providing a sensor for detecting the ambient brightness (intensity of illumination on the side of the display portion) and by varying the brilliance of the display depending upon the ambient brightness.
When an EL (Electro Luminescence) display device is used for a display, a voltage and a current are fed to the EL elements depending upon the brilliance to effect the gradation display. However, the EL element is deteriorated due to an aging factor, and the degree of deterioration differs depending upon three primary colors of RGB. To obtain a multi-color display on the EL display device, therefore, the brilliance of three primary colors of RGB must be adjusted to control white balance. In this case, there has been proposed a method of expressing an image with a correct gradation of a desired color by bringing the brilliance of other colors into agreement with a color that has been deteriorated, i.e., a color having a low brilliance.
Similarly, when a liquid crystal display device is used for a display, the brightness of display differs depending upon the brilliance of back light. Therefore, there has been proposed a method of producing a display with a predetermined brightness at all times by adjusting the brilliance of the display depending on the deterioration of back light due to aging.
As compared to a portable telephone which cannot be folded, the foldable portable telephone has many advantages, i.e., (1) small in size and is portable, (2) the display portion is provided in a first housing, and the operation portion is provided in a second housing enabling the display portion to possess a large display area and to display an increased amount of information at one time, and (3) induces less erroneous operation during the standby.
In the conventional foldable portable telephone, information is displayed on a display being triggered by the reception of electromagnetic waves, depression of operation buttons or opening/closure of the housing (patent document 1).
(Patent document 1) JP-A-2002-101160 (page 6, FIGS. 2 and 7)